[hamradio-commits] [dump1090] 124/389: Improve DF field error correction speed

[Matthew Ernisse]

This is an automated email from the git hooks/post-receive script.

mernisse-guest pushed a commit to branch master
in repository dump1090.

commit c81498a0d02d55bfa7890bed67d245fba4139ee4
Author: Malcolm Robb <Support at ATTAvionics.com>
Date:   Tue Apr 30 10:35:52 2013 +0100

    Improve DF field error correction speed
    
    Use bitmasks to speed up the detection of valid and invalid ICAO DF
    field types.
---
 dump1090.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/dump1090.c b/dump1090.c
index bbe5519..4929344 100644
--- a/dump1090.c
+++ b/dump1090.c
@@ -1977,18 +1977,25 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
         if      (msglen > i) {msglen = i;}
         else if (msglen < i) {msglen = 0;}
 
+        //
+        // If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
+        // Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
+        // message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
+        // if the new value is ICAO defined, then update it in our message.
         if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
-            // We guessed at one of the message type bits. See if our guess is "likely" 
+            // We guessed at one (and only one) of the message type bits. See if our guess is "likely" 
             // to be correct by comparing the DF against a list of known good DF's
-            int DF = ((theByte = msg[0]) >> 3) & 0x1f;
-            if ( (DF !=  0) && (DF !=  4) && (DF !=  5) && (DF != 11) 
-              && (DF != 16) && (DF != 17) && (DF != 18) && (DF != 19) && (DF != 20) && (DF != 21) && (DF != 22) && (DF != 24) ) {
-                // Other DF values are probably errors. Toggle the bit we guessed at and see if the resultant DF is more likely
-                theByte ^= theErrs;
-                DF       = (theByte >> 3) & 0x1f;
-                // if this DF any more likely??
-                if ( (DF ==  0) || (DF ==  4) || (DF ==  5) || (DF == 11) 
-                  || (DF == 16) || (DF == 17) || (DF == 18) || (DF == 19) || (DF == 20) || (DF == 21) || (DF == 22) || (DF == 24) ) {
+            int      thisDF      = ((theByte = msg[0]) >> 3) & 0x1f;
+            uint32_t validDFbits = 0x017F0831;   // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
+            uint32_t thisDFbit   = (1 << thisDF);
+            if (0 == (validDFbits & thisDFbit)) {
+                // The current DF is not ICAO defined, so is probably an errors. 
+                // Toggle the bit we guessed at and see if the resultant DF is more likely
+                theByte  ^= theErrs;
+                thisDF    = (theByte >> 3) & 0x1f;
+                thisDFbit = (1 << thisDF);
+                // if this DF any more likely?
+                if (validDFbits & thisDFbit) {
                     // Yep, more likely, so update the main message 
                     msg[0] = theByte;
                     Modes.stat_DF_Type_Corrected++;

-- 
Alioth's /usr/local/bin/git-commit-notice on /srv/git.debian.org/git/pkg-hamradio/dump1090.git